Abstract Huntington?s disease is caused by a trinucleotide expansion mutation that extends a polyglutamine (polyQ) tract in the huntingtin (HTT) protein, but the cellular mechanism of its toxicity, especially the inverse correlation of onset with polyQ length, is not understood. HTT interacts with polycomb repressive complex 2 (PRC2), a histone methyltransferase (HMT) which deposits cellular H3K27trimethylation and interestingly, expanded-polyQ HTT enhances PRC2 activity in a polyQ-length-dependent manner. We found that Htt null mouse ESCs have altered, complex HMT activity signatures, and immortalized homozygous mutant striatal cells show higher HMT activity compared to wild-type, leading us to hypothesize that there are additional HMTs that interact with HTT in a polyQ-length-dependent manner. We will first separate cellular HMTs in nuclear lysates by their native complex sizes and determine HMT activity in each fraction in the absence or presence of recombinant full-length huntingtin with various polyQ lengths. Then, we will identify key HMTs in the peaks that are influenced by the size of the polyQ tract in HTT by LC-MS/MS, followed by a secondary assay with nucleosomes having histone mutants which has Ala on the target Lys residue . In order to characterize the functional effects of key HMTs, we will assess the consequence of their systematic knockdown in HD iPSC-derived neuronal cells using our functional quantitative assays. Finally, we will use DNA aptamers to test how HTT interacts with HMTs. By identifying the interaction between novel HMTs with mutant HTT, we will contribute to the understanding of the role of HMTs on altered epigenetic regulation in HD.